1. Field of the Invention
The field of endeavor of the present invention is an apparatus and method for the transmission of digital data utilizing a carrier having the characteristics of a sideband with a very narrow bandwidth so as to reduce the bandwidth of the principal energy bearing portion of the transmitted radio frequency spectrum.
2. Description of the Prior Art
Single sideband transmission of information without a carrier is well known. For the transmission of digital data utilizing a single sideband, some form of baseband data encoding is generally required to reduce the bandwidth. The encoded data is then applied to a single sideband modulating device, which suppresses the carrier and removes one of the sidebands by a filtering or phasing method.
U.S. Pat. Nos. 4,742,532, 5,185,765 and 5,930,303, issued to the present inventor are representative of the prior art. A xe2x80x98PCTxe2x80x99 application published as WO 99/23754 (U.S. Ser. No. 98/23140) is an international filing of the ""303 patent. U.S. patent application Ser. No. 09/612,520 is directed to a method related to the ""303 patent.
The method described in the first and second of the above patents results in some frequency spread in the transmitted spectrum. The latter patent describes a method known as VMSK that results in a single frequency spectral line with phase changes so slight that they are not visible on spectrum analyzers. The VMSK method results in a very high spectral efficiency that enables very high data rates to be transmitted in a very narrow bandwidth. It was also found that it is not necessary to restore the suppressed carrier to detect the signal.
Since the modulation and filtering produce a spectrum of a single frequency, special very narrow band filters are required to pass the narrowest bandwidth possible to remove undesirable spectral components in the transmitter and to reduce the noise bandwidth in the receiver. In the ""303 patent, phase reversals of the carrier are utilized together with a balanced modulator to remove the carrier. The phase reversal periods are made as nearly even as possible in order to reduce undesirable spectral components. When only one sideband is transmitted, after passing through the narrow bandwidth filter, the observed signal on the oscilloscope does not show the phase reversals at nearly equal time periods. Instead, the signal shows a complete phase rotation through 360 degrees at the transition periods of the phase reversing signals. The duration of this phase rotation is three to four cycles of the sideband frequency, which normally has 60 to 90 cycles per bit period. For the remainder of the bit period, the sideband frequency is constant in frequency and phase.
All of the useful modulation is contained in the brief phase rotation period. For the remainder of the bit period there is a constant signal that can be used to establish a reference. In the prior art, the data clock and the RF frequency are not necessarily numerically related so that there is a phase crawl or difference among the various crossings periods. This results in an inconsistent detected output, which varies in rise time and amplitude.
There is a need for a modulation method that will synthesize the sideband of the prior art by itself, with brief phase reversals at timed intervals representing digital ones or zeros. There is a need for a modulation method of this kind, and that does not require the use of a carrier similar to what has been done; yet without the customary single sideband processing with its drawbacks. That is, phase crawl or difference among the various crossings periods needs to be reduced or eliminated. This would fill an end goal or need for a method that results in a consistent detected pattern in rise time and amplitude.
Several complementary objects of the present invention involve elements in a system that comprises the invention. Hence, one object of the invention is to provide a detection means that will detect the simulated sideband of the invention without the restoration of a carrier separated in frequency from the sideband by xc2xd the data rate. Another object of the invention is to provide a signal processing means that will result in no apparent loss of signal power. Still another object of the invention is to provide a signal that can withstand the degradation caused by multipath interference and fading.
The present invention fills these needs as can be seen in the further description below.
The wireless digital transmission and receiving method of the present invention combines phase reversal keying with pulse position modulation. The invention implements pulses that are of extremely short duration to indicate ones and zeros. These pulses can be as narrow as one cycle of the carrier frequency. As such, they often appear as missing cycles or pulses in a sequence of carrier cycles. The method creates a broad sinx/x spectrum with a principal power peak at the modulated frequency and numerous weaker peaks of varying frequencies and peak levels. The time duration of these smaller peaks is such that they have negligible mean power levels. The weaker peaks also do not cause measurable interference with other communications devices. Also, the smaller peaks can have amplitudes far below the noise level of the system. Hence, the smaller peaks are not an important component of the signal.
It is well known to those skilled in the art that all modulation energy is concentrated in the sidebands. It can be shown that the spectrum of the present invention represents a sideband only and not a carrier with two sidebands. It can also be shown that this sideband is comprised of a single frequency that does not change in frequency or phase for most of the bit period. The modulation occurs as a phase reversal of one cycle, or as the absence of one cycle in a series of cycles. The resulting xe2x80x9cmissing pulsesxe2x80x9d or xe2x80x9cmissing cyclesxe2x80x9d are detected to indicate digital ones and zeros in a signal of fixed frequency. It is possible to detect these xe2x80x9cmissing pulsesxe2x80x9d or xe2x80x9cmissing cyclesxe2x80x9d at preset time periods.
The method is extremely resistant to multipath interference, since the weaker path is a signal at the same single frequency as the stronger path, but of different phase. The phasors of the two paths add to produce large detected outputs for the principal path and weak outputs at a different time for the reflected path. Thus, this weak response can be time gated out of the resulting signal.
In a practical first embodiment of the invention, a circuit is used to create very narrow pulses utilizing pulse position modulation to distinguish ones and zeros. The narrow pulses are used in a phase reversing device, such as an XOR gate or balanced mixer, to cause the reversal of one cycle out of a stream of many cycles. The resulting spectrum has a principal peak at the modulated frequency and many minor peaks spread at bit rate intervals. The minor peaks have power levels proportional to the time duration of the pulses of each phase. The mean power of the minor peaks is low. Therefore, it is not normally required to provide bandpass filtering at the transmitter.
In a second embodiment, an AND gate is used to remove one cycle from the modulated frequency so that xe2x80x9cmissing cyclexe2x80x9d modulation results. The result is the same as that for phase reversal.
In all embodiments, the timing of the narrow pulse and the start of the cycles of the modulated frequency can be made to coincide with each other so that the detected output has a uniform rise time and amplitude.
The present invention may be used in conjunction with any number of elements in a system. For example, in a system, the receiving apparatus is made to correspond with the method and device of the present invention. The receiving apparatus is comprised of a special very narrow bandwidth filter, having almost zero group delay, in combination with a limiter, synchronous detector, and pulse position decoder. The synchronous detector is locked in frequency and phase to the pulse modulated frequency.
In such a system, the pulse position decoder recovers the data clock and includes gating circuitry to turn the circuit off except at the expected time of a phase change arrival.
In particular, the invention is defined as a method of transmitting and receiving digital data in a wireless communications system. In transmitting the method comprises the steps of phase reversal keying a carrier frequency of a signal used for transmitting and/or receiving digital data, and pulse position modulating the signal by generating two pulses having opposite phases, namely generating a primary pulse of the two pulses taking a majority fraction of a bit period, and generating a secondary pulse of the two pulses having a minority fraction of a bit period. The time position of the secondary pulse carrying modulation information.
The step of generating the secondary pulse generates a pulse having a duration equal to or less than 3 to 4 cycles of the carrier frequency. In a preferred embodiment the step of generating the secondary pulse generates a pulse having a duration equal to one cycle of the carrier frequency.
The step of phase reversal keying a carrier frequency of a signal comprises reversing the phase of the signal beginning at a data clock boundary, and continuing for 1 to 3 cycles to represent a digital one. Again in the preferred embodiment the step of phase reversal keying a carrier frequency of a signal comprises reversing the phase of the signal beginning after a short delay following a data clock boundary to represent a digital zero.
In another embodiment the step of phase reversal keying a carrier frequency of a signal comprises reversing the phase of the signal beginning at a data clock boundary to represent a digital zero and reversing the phase of the signal after a short delay after a data clock boundary to represent a digital one.
In a preferred embodiment the step of phase reversal keying a carrier frequency of a signal comprises providing a maximum acceptable duration between the phase reversals by maintaining the phase of the carrier in a undisturbed state for substantially the duration of the number of carrier frequency cycles left in the bit period after the brief reversal. For example the step of pulse position modulating the signal comprises maintaining the frequency and phase of the first primary pulse for at least 95% of the bit period to provide the maximum acceptable undisturbed duration.
The step of pulse position modulating the signal results in a spectrum containing a multiplicity of sinx/x frequency peaks. The spectrum has a single maximal frequency peak at the carrier frequency and numerous minor low level frequency peaks of differing amplitudes separated from the maximal frequency peak at intervals equal to the bit rate. In the preferred embodiment the step of pulse position modulating the signal comprises creating the maximal frequency peak to contain approximately 99% of the total radiated power of the spectrum. Alternatively, the step of pulse position modulating the signal comprises creating the lower level frequency peaks spread such that the individual mean power of the lower level frequency peaks is less than one millionth of the maximal frequency peak at the carrier frequency.
The step of phase reversal keying a carrier frequency of a signal comprises representing a single RF cycle phase reversal as a missing pulse while retaining pulses for all other RF cycles of the bit period.
The step of pulse position modulating the signal comprises creating a spectrum with a main energy peak characteristics of a single sideband related to a phantom carrier in which the main energy peak is synthesized into a single side band, such that the signal no longer has other sidebands relative to the main energy peak. The step of pulse position modulating the signal comprises creating the minor frequency peaks with a short duration much less than one bit period, so that the minor frequency peaks thus do not pass through a conventional filter, and so that the maximal frequency peak passes the conventional filter as a single frequency without indication of modulation.
In receiving the method further comprises the step of demodulating the signal in which a maximal frequency peak is modulated by received signals from a principal transmission path and at least one echo path during a time interval which is much less than a bit period, so that the principal transmission path and the echo path are detected as having separable phase reversal transition times, thereby reducing multipath interference.
In receiving the method further comprises the step of demodulating the signal in which the minor frequencies peaks are either lower in amplitude than the system noise or are removed by filtering.
The invention is also defined as a communications system for transmitting digital data comprising: encoding means for phase reversal keying a carrier frequency of a signal used for transmitting and/or receiving digital data; and modulating means for pulse position modulating the signal by generating two pulses having opposite phases, namely for generating a primary pulse of the two pulses occupying a majority fraction of a bit period, and for generating a secondary pulse of the two pulses occupying a minority fraction of a bit period, the position of the secondary pulse carrying the modulation information.
The system includes a system clock and the encoding means comprises a narrow pulse width generator responsive to a data input by creating a pulse having no delay relative to a clock to represent a digital one, and the pulse width generator responsive to a data input by creating a pulse having a short delay after the relative to the clock to represent a digital zero.
In one embodiment only pulses representing a digital one are transmitted or encoded by the encoding means.
The modulating means comprises a balanced modulator, wherein the balanced modulator causes phase reversals, but does not suppress a carrier. The modulating means may be an XOR gate or other commonly used balanced modulator.
The system also receives the signal and further comprises filtering means. The filtering means comprises a monopole crystal with a high Q. The monopole crystal is caused to continuously resonate at the carrier frequency, such that the filtering means cannot reverse a resonant phase in the short time period of the narrow pulse width, so that the filtering means passes a modulated signal without following the phase change of the secondary pulse, and so that the filtering means rejects signals which have a frequency which differs from the carrier frequency.
In receiving the system further comprises a detecting means. The detecting means comprises a synchronous detector locked to the single transmitted frequency. The detecting means generates a spiked output only during a time of the pulse generated by the narrow pulse width generator. The detecting means typically comprises an XOR gate. The XOR gate responds to phase reversals or to missing pulses in the signal.
As a receiver the system further comprises a decoding means, a receiver data clock, and an output circuit. The decoding means produces a spiked output at the time of a digital one. The spiked output resets the receiver data clock and sets the output device to indicating that a digital one has been received.
The invention now having been briefly summarized, its various features and embodiments may be visualized in the following drawings and illustrations, where like elements are referenced by like numbers.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.